The present invention relates in general to dynamic frequency equalization and more particularly concerns novel apparatus and techniques for controlling the frequency response of a powered loudspeaker system as a function of level without introducing undesired boominess in voice reproduction.
Dynamic equalization is disclosed in U.S. Pat. No. 4,490,843, incorporated by reference herein. The patent explains that the sensitivity of human hearing to bass frequencies (relative to other frequencies) is less at lower volume levels than it is at higher volume levels. To compensate for this difference, many audio amplifying systems include a loudness control that boosts the bass and treble response at lower volume levels. Conventional loudness contours are based upon Fletcher-Munson equal-loudness curves which relate the frequency response of human hearing to the level of the sound being heard.
In the above-referenced patent, it was discovered that even though the Fletcher-Munson curves predict that one should boost the frequencies between 200 and 500 Hz, very satisfactory musical performance can be obtained even if only the frequencies below 200 Hz are boosted. Also, by limiting the boost to frequencies below 200 Hz, undesirable effects such as boominess in the reproduction of voices were reduced. There was discovered and disclosed a family of frequency contours which vary with volume control setting to improve the perceived low level music reproduction without degrading voice reproduction. Essentially, as the volume control is changed to decrease the mid and high frequency level by two decibels, the system reduces the level of a narrow band of frequencies centered about the low frequency cutoff of the loudspeaker by only one decibel. The family of curves shown in FIG. 6 of the patent is typical.
Dynamic equalization, as described above, provides full, rich sound with pleasing bass at any listening level. Conventional loudness controls (such as those that provide an inverse Fletcher-Munson curve) can not provide this effect, because in addition to boosting deep bass, they also boost mid-bass, causing male voices and instruments like cellos to sound unnaturally boomy.
Automatic dynamic equalization is disclosed in U.S. Pat. No. 4,739,514, incorporated by reference herein. This patent describes apparatus that achieves some of the benefits as the invention of U.S. Pat. No. 4,490,843, but for systems in which the volume control is not a part of the system. The Automatic Dynamic Equalization circuit senses the level of audio signal applied to its input. As the input level drops by two decibels, the bass portion of the circuit output is boosted so that its level drops by only one decibel. Such a circuit might be used in, for example, after-market car stereo systems and powered loudspeaker systems, such as the commercially available BOSE ROOMMATE system, or any other system having a conventional volume control preceding the power amplification stages.
Although automatic dynamic equalization can provide some of the benefits of dynamic equalization, it can also create artifacts. With automatic dynamic equalization, the bass boost depends on the loudness of the music. Thus, the pianissimo portion has much more bass boost than the fortissimo portion. While the resulting artifact is acceptable, and even pleasant for music with limited dynamic range (such as pop music), for music with wide dynamic range (such as classical), the artifact can negatively affect the perceived quality of reproduction.
The commercially available Bose Lifestyle music system, incorporated herein by reference, describes a program signal processing system which allows several program signal monitoring locations to independently access a centralized group of program signal sources. With this system, each monitoring location has independent access to all of the available program sources. In an audio application of this invention, the speakers which connect to the system may be wired directly to a central amplifier, but are preferably wireless and contain their own amplifiers.
When using dynamic equalization according to the invention, the frequency of boost is normally set to the low frequency cutoff of the system (which usually corresponds to the low frequency cutoff of the loudspeaker). In addition, the amount of boost is adjusted according to the absolute dB (SPL) being reproduced (which depends on the setting of the volume control, the sensitivity of the loudspeaker, and the acoustics of the listening environment). During installation, the gain and offset parameters of the dynamic equalization circuit are adjusted to account for the loudspeaker sensitivity and the acoustics of the listening environment. Thereafter, in operation, the circuit adjusts the boost level depending on the adjusted parameters, and the current level of the volume control setting. Because the boost level does not depend on position of a manual switch or the actual instantaneous loudness of the music, the amount of bass boost is always correct for the loudness level selected by the listener. For example, the fortissimo and the pianissimo portions of a symphony are reproduced with the same (correct) amount of bass boost.
According to the invention, there is a powered loudspeaker system having a cabinet. The cabinet has at least one electroacoustical transducer and a power amplifier coupled to the electroacoustical transducer. Dynamic equalization circuitry is coupled to the power amplifier and has circuit parameters for establishing a predetermined desired dynamic equalization for the electroacoustical transducer in the cabinet when normally positioned in a listening room.